Assessment of the distribution of radioactive contamination is an essential process for safe decommissioning. Gamma cameras are widely used to investigate the hot spots of radioactive materials. The 3D information of the radioactive contamination is required to reduce the occupational exposure and the radioactive waste. The purpose of this study is to design a multi-pinhole collimator for the 3D gamma ray imaging. The collimator was designed by Monte Carlo simulation and the performance was evaluated by lab test. The collimator consists of four cone-shaped pinholes, tungsten aperture, and lead septa. The acceptance angle and the source to detector distance were 40 degrees and 15 cm, respectively. A number of 2D images were obtained by the linear motion of the gamma camera and the 3D images were reconstructed by filtered back-projection algorithm. As a result, the experimental results were within 2 % of the expected values.